A surge in Covid-19 hospital admissions driven by the BA.5 subvariant of Omicron, accompanied by the inability of vaccines to prevent reinfection, has prompted health policymakers to rethink their approach to boosters.

US regulators last week recommended changing the design of vaccines to produce a new booster targeting Omicron — the first change to the make-up of shots since their introduction in late 2020. Research into immune imprinting, whereby exposure to the virus via either infection or vaccination determines an individual’s level of protection, is now driving the debate over the make-up of Covid-19 vaccines.

Immunologists say that, more than two years into the coronavirus pandemic, people have acquired very different types of immunity to the Sars-Cov-2 virus, depending on which strain or combination of strains they have been exposed to — leading to big differences in Covid-19 outcomes between individuals and countries.